Introduction

As we have seen earlier in the chapter of Principle of UV-Vis Spectroscopy, absorption of a particular wavelength of light depends upon the π-electron system of the molecule. The more the conjugation of the π-electron system within the molecule, the higher the wavelength of light it can absorb. Robert Burns Woodward and Louis Fieser put down a set of rules which allows one to calculate the wavelength of maximum absorption (λmax) for a molecule empirically. These sets of rules to calculate the wavelength of maximum absorption or λmax of a compound in the ultraviolet-visible spectrum, based empirically have been called the Woodward-Fieser rules or Woodward’s-rules.

These sets of article aims to guide the student on how to use these rules to calculate the wavelength of maximum absorption or λmax for different systems. The following pages are listed for the readers.

Note: Numerical values for Woodward-Fieser rules differ slightly from one textbook to another. We have tried to compile an extensive list of numerical values from online resources, textbooks and journal articles based on the popularity of the number. It is recommended that you learn on how to apply the values for the contributors and then follow the values given in a text book recommended by your teacher, or use our values. We believe that learning how to apply the rules is more essential than actually getting the exact answer. Other’s opinions may vary.

Woodward-Fieser Rules for Calculating the λmax of Conjugated Carbonyl Compounds

Woodward-Fieser rules can be extended to calculate the λmax of α,β-unsaturated carbonyl compounds. In a similar manner to Woodward rules for dienes discussed previously, there is base value to which the substituent effects can be added and the λmax can be calculated using the formula:

Table 2: Gives the values for the influence of different chromophores in conjugated carbonyl systems as per Woodward-Fieser rules. The usage of these will become more evident in the examples which follow.

Class

Chromophore

Influence to λmax

Core Chromophore Base Value

α,β-unsaturated carbonyl
compound

If R = -H (α,β-unsaturated aldehyde)

If R = -Alkyl (α,β-unsaturated ketone)

If R = -OR (α,β-unsaturated ester)

+ 210 nm

+ 215 nm

+ 195 nm

Cyclopentenone

+ 202 nm

Cyclohexenone

+ 215 nm

α,β-γ,δ-diene carbonyl
compound

If R = -H ( α,β-γ,δ-diene aldehyde)

If R = -Alkyl ( α,β-γ,δ-diene ketone)

If R = -OR ( α,β-γ,δ-diene ester)

+210 + 30 =+240 nm

+215 + 30 = +245 nm

+195 + 30 = +225 nm

Substituent Effects
at α-position

-R (Alkyl group)

+ 10 nm

-OR (Alkoxy group)

+ 35 nm

-Cl (Chloro group)

+ 15 nm

-Br (Bromo group)

+ 25 nm

-OH (alcohol/hydroxyl)

+ 35 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

Substituent Effects
at β-position

-R (Alkyl group)

+ 12 nm

-OR (Alkoxy group)

+ 30 nm

-Cl (Chloro group)

+ 12 nm

-Br (Bromo group)

+ 30 nm

-OH (alcohol/hydroxyl)

+ 30 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

-SR (Sulfide)

+ 85 nm

-NR2 (Amine)

+ 95 nm

Substituent Effects
at γ and δ-position

-R (Alkyl group) (both γ and δ)

+ 18 nm

-OC(O)R (Acyloxy/Ester)
(both γ and δ)

+ 6 nm

-Cl (Chloro) (both γ and δ)

+ 12 nm

-Br (Bromo) (both γ and δ)

+ 30 nm

-OH (alcohol/hydroxyl group)
(only γ)

+ 50 nm

-OR (Alkoxy group) (only γ)

+ 30 nm

Further π-conjugation

– C=C (double bond
extending conjugation)

+ 30 nm

– C6H5 (Phenyl group)

+ 60 nm

Other Contributors

Exocyclic Double Bond

+ 5 nm

Homoannular cyclohexadiene

+ 35 nm

Solvent Effects

1] Water

– 8 nm

2] Methanol/Ethanol

– 1 nm

3] Ether

+ 6 nm

4] Hexane / Cyclohexane

+ 7 nm

Let us discuss each of the above values and when to apply them in greater detail with examples:

Core Chromophores With Base Values

As Woodward and Fieser have listed, α,β-unsaturated carbonyl compounds have a range of influence on the λmax of the molecule depending upon:

2] If the core is a part of a cyclic ring. For example, cyclopentenone contribution is 202 nm while cyclohexenone is 215 nm.

3] If the conjugation is extended to γ,δ-positions to form dienes. For example, in such cases, a simple addition of 30 nm to the base value of the α,β-unsaturated carbonyl compound gives appropriate estimates to the observed influences.

Note- In cases of α,β-γ,δ-diene carbonyl compounds like those shown above, the extended conjugation at the α,β-γ,δ-positions is accounted for in the base value of the core chromophore and need not be added separately. If however there is another substituent at the α,β-γ,δ-positions, then you must add an additional + 30 nm for each.Alsothe bond shown as β-γ is not counted as β substituent but as a part of the core chromophore and need not be added separately.

Substituent Effects

According to Woodward, in case of α,β-unsaturated carbonyl compounds, the location of the substituent is significant in determining the influence on the wavelength of maximum absorption. Substituents can be located on either α, β positions. If the conjugation is extended to γ and δ positions, then substitutions at these position also play a vital role in determining the practical λmax.

1] Substituents at α-Position

As we can see the from table 3 below the effect of different substituent when placed on the α-position.

Table 3: Effect of substituents on the α-position of α,β-unsaturated carbonyl compounds

Substituent

Influence

-R (Alkyl group)

+ 10 nm

-OR (Alkoxy group)

+ 35 nm

-Cl (Chloro group)

+ 15 nm

-Br (Bromo group)

+ 25 nm

-OH (alcohol/hydroxyl)

+ 35 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

2] Substituents at β-Position

As we can see the from table 4 below the effect of different substituent when placed on the β-position.

Table 4: Effect of substituents on the β-position of α,β-unsaturated carbonyl compounds

Substituent

Influence

-R (Alkyl group)

+ 12 nm

-OR (Alkoxy group)

+ 30 nm

-Cl (Chloro group)

+ 12 nm

-Br (Bromo group)

+ 30 nm

-OH (alcohol/hydroxyl)

+ 30 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

-SR (Sulfide)

+ 85 nm

-NR2 (Amine)

+ 95 nm

3] Substituents at γ and δ-position

As we can see the from table 5 below the effect of different substituent when placed on the γ or δ position.

Table 5: Effect of substituents on the γ or δ position of α,β-γ,δ-diene carbonyl compound.

Since carbonyl functional groups have polarity, solvents play an important role in how the electronics of the structure play out. The rules are simple and straight forward:

Solvent

Influence

Water

– 8 nm

Methanol/Ethanol

– 1 nm

Ether

+ 6 nm

Hexane / Cyclohexane

+ 7 nm

3] Homoannular Cyclohexadiene

In a special case where you have α,β-γ,δ-diene carbonyl compound and both the double bonds are present within one ring system you get a homoannular or homocyclic cyclohexadiene carbonyl compound. In such a case you must add an additional 35 nm to the system.

Sample Problems to use Woodward-Fieser Rules to Calculate λmax of Conjugated Carbonyl Compounds

Author Unknown. Michigan State University – Department of Chemistry. Woodward-Fieser Rules for Calculating the π to π* λmax of Conjugated Carbonyl Compounds. [Online]http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/uvspec.htm (Accessed on July 27, 2012).

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4 Comments

Thomas

May 7, 2015

When you discuss the Woodward-Fieser rules for unsaturated carbonyl compounds, you give a negative value for the solvent shift in polar solvent (e.g., -8 nm for water) and a positive solvent shift for unpolar solvents (+7 nm for hexane). About half of all textbooks and web-resources give the opposite sign for the solvent shift (common value: +8 nm for water, -11 nm for hydrocarbon solvents). Maybe it would be worthwhile to check the original literature to ensure the values are correct.

Hi Thomas,
I went through literature and texts to find information about the reasoning for the -8nm for water and a +7nm for hexanes. It seems that pi->pi* transitions undergo bathochromic (red shifts or increase in wavelengths), while n->pi* transitions undergo hypsochromic (blue shifts or decrease in wavelength) when solvent polarity is increased. In case of carbonyl compounds, since a n->pi* transition governs the UV spectrum, it is expected that you will observe a blue-shift (hypsochromic) when you increase the polarity of solvent. Thus water would yield a -8 nm, while hexane would give a +7 nm shift. Please let me know which books give opposite values and the page so that I can check it.